players can choose any two numbers they like and change them to a number abs(x-y). in his turn, he chooses numbers randomly, each numbers left on the table has the same possibility to be chosen. 他要选两个数，随机怎么定义，是每对(x, y)被选中的概率相同还是先随机选一个，再随机选一个？ 比如我有 1 2 3 选中1 2的概率是1 / C(3, 2)还是 1 / 3 * 1 / 2 ?

没事了。。。好像都一样。。。